Experimental autoimmune encephalomyelitis (EAE) is a CD4 T cell-dependent model for the human disease multiple (MS). Several antigen presenting cells (APCs) coordinate and regulate CD4 T cell function during immune responses. B cells likely have multiple roles in the pathogenesis of MS, with their capacity to function as APCs and drive auto-reactive CD4 T cell responses gaining more recognition. However, questions remain as to the importance of antigen presentation in the role for B cells in EAE and MS. We have designed a new tool in which individual subsets of APCs are capable of conditionally expressing MHCII in vivo. We successfully targeted a stop sequence flanked by loxP sites to the MHCII ? chain locus in mice in order to utilize the Cre/loxP system for conditional expression of MHCII. Successful conditional manipulation was achieved using B lineage-specific Cre mice, restricting expression of MHCII to B cells. B cell expression of MHCII alone was sufficient to support inflammatory demyelination of the central nervous system (CNS) mediated by encephalitogenic CD4 T cells, but only when B cells were highly efficient at recognizing target antigen. Based on these observations, we hypothesize that the process by which B cells coordinate antigen- specific CD4 T cell autoimmune destruction of myelin is dependent upon B cell localization to the CNS and efficient capture of target antigen during the propagation of EAE. We aim to: 1) Determine the requirement for antigen-specific humoral responses during EAE mediated by B cell antigen presentation; 2) Determine the requirement for B cell antigen presentation in the CNS compartment during EAE; and 3) Determine the effect of B cell depletion on dendritic cell (DC) antigen presentation in EAE and MS. While the proposal herein utilizes a novel system to explore the role of B cell antigen presentation during EAE, this powerful murine genetic system is optimally designed to investigate the cellular contributions by various APCs in different models of immunity and autoimmunity. Furthermore, studies pursuing functional alterations in DCs from MS patients receiving B cell depletion therapy will bring into context the relation between antigen specific B cell regulation of CD4 T cell function and B cell therapies used to treat patients with MS. These studies will provide mechanistic understanding of immune regulation by B cells in MS and offer potential for future design of optimal B cell and CD4 T cell therapeutics.